1. Field of the Invention
The invention relates to a medical implantable lead of the type being adapted to be implanted into a human or animal body and attached with a distal end to an organ inside the body, and having a helix of a helical wire in the distal end which is adapted to be screwed into the organ, wherein the lead, in addition to the first helix, also comprises a second helix of a helical wire, the second helix having the same pitch and being intertwined with the helical wire of the first helix and which, upon rotation of the first helix, will be rotated and screwed into the tissue.
The invention also relates to a method for attaching a medical implantable lead to an organ inside a human or animal body, by screwing a helix of a helical wire into the tissue of the organ.
The invention is primarily adapted for attaching the tip of a cardiac electrode into cardiac tissue for connecting e.g. a pacemaker or defibrillator to a heart. However, the invention is also applicable for attaching other types of electrodes to arbitrary organs in a human or animal body. Accordingly, everywhere in the following description and claims where reference is made to attachment of an electrical lead to a heart, it is to be understood that it may concern also other types of organs where applicable.
2. Description of the Prior Art
Various types of cardiac stimulation devices, such as cardiac pacemakers, have one or more electrode leads connected at one end, commonly called the proximal end, to the implanted stimulator housing, and having an opposite end, commonly called the distal end, which is implanted so as to interact with cardiac tissue. The attachment of the distal end of a cardiac lead to the cardiac tissue is commonly referred to as fixing the lead, or lead fixation.
Typically, the cardiac lead carries an electrode at or near the distal end thereof, and this electrode, by fixing the electrode lead to the cardiac tissue, must be placed in good electrical contact with the cardiac tissue. The distal end of the cardiac lead also must be reliably mechanically affixed to the cardiac tissue, so that beating of the heart, and other physical movements of the subject, will not cause dislodgement of the lead.
Many types of lead fixation techniques and structures are known, one of the most common being to configure the tip electrode of the electrode lead to be in the form of a helix or corkscrew, which is then screwed into the cardiac tissue during the implantation procedure.
An example of such a lead with a helical configuration is described in U.S. Pat. No. 6,687,550. As in the arrangement described in this patent, the helical electrode during implantation is commonly retracted inside of a protective covering or header, and upon reaching the implantation site at the cardiac tissue, the helix is actively caused to protrude from the covering and is screwed into the cardiac tissue.
United States Patent Application Publication No. 2003/0073972 discloses a catheter assembly for implanting a helical fixation device into the myocardium. The catheter assembly has a fixation wire in the form of a helix that is securely fastened to the distal end of the catheter. A drug delivery coil or helix is nested inside the flights (turns) of the fixation helix, but the catheter prevents proximal rotation of the drug delivery coil. When the surgeon screws the fixation coil into the heart wall, the drug delivery helix is driven along with the fixation helix, but when the surgeon unscrews the fixation helix from the heart wall, the drug delivery helix remains in place in the cardiac tissue. The catheter also has a centrally located hollow straight needle that extends through the central axis of the two helices, and is used for penetrating the myocardium when the helices are screwed into the myocardium. The two helices have the same pitch and helix diameter, so as to permit the drug delivery coil to be nested within the fixation helix with a sliding fit.
From U.S. Pat. No. 7,212,870 a helical fixation device is known that has two helices having the same length, diameter and pitch, which are formed of helical wires and are intertwined with each other such that the wires are displaced 180° in relation to each other. The fixation device is adapted to function as a bipolar electrode, wherein one helix will be a cathode whereas the other an anode. One disadvantage with a fixation device like this is that the impedance between each helix and the tissue will be small. A small impedance will lead to increased energy consumption and hence more frequent replacement of the implanted electronic devices due to discharged batteries. In some applications measures may have to be taken to avoid this problem, such as partly insulating of the helices as is disclosed in the document, which will lead to increased costs for manufacturing.
In order to reduce the size of implantable electrode leads, and to reduce the lead impedance, there is a trend to form the fixation helix from thinner wire than in the past, as well as to reduce the outer diameter of the fixation helix. This trend has resulted in side-effects, such as high acute capture thresholds and poor mechanical fixation of the distal end of the lead. It will also lead to an increased risk that the thin wire helix may easily start to rotate in the myocardium behind the endocardium at the surface of the heart wall if the helix is over-rotated. This will tear the myocardium around the helix, which will cause unnecessarily injury to the heart, which causes poor connection, electrically as well as mechanically, and increased capture threshold between the helix and the tissue.